1. Field of the Invention
This invention relates generally to dryers for printed materials and the like, and in particular to a dryer for printed webs utilizing infrared radiant heat and forced air.
2. Description of the Prior Art
In the printing field, dryers are often employed to shorten the drying time of inks and dyes applied to various materials such as paper and paperboard stocks, metallic foils, plastic films and cloth. Such materials may be printed in the form of either individual sheets or in continuous webs.
Printing inks and dyes generally contain solvents, varnishes and pigments which evaporate after sufficient exposure to ambient atmospheric conditions. However, in printing operations it is frequently desirable to hasten the drying process so that the printed materials may be stacked, rewound, cut, folded or otherwise further processed without smearing the ink or dye thereon.
One method of reducing the drying time involves the application of anti-offset powder to the freshly printed materials. The powder absorbs the solvents in the ink or dye to prevent smearing when the materials are further processed. However, the powder and the equipment for its application increase the printing costs. Also, drying powder tends to disperse widely around the area of the printing operation, interferes with equipment, and must be periodically cleaned up whereby the printer incurs additional maintenance costs by its use.
A variety of different dryers and ovens have been devised for applying heat to freshly-printed material. For example, tunnel ovens have heretofore been provided for transferring heat to webs of printed material. In tunnel ovens, the heat exchange occurs primarily by convection. However, tunnel ovens tend to be relatively large and are therefore unsuitable where floor space is limited. Also, they are relatively inefficient because much of the heat generated thereby is absorbed by the atmosphere and the surrounding structure instead of the printing material and the ink thereon.
Radiant heaters are generally more efficient in drying operations than convective heaters because radiant energy is transmitted through the atmosphere with very little loss and is absorbed by liquids and solids. Therefore, relatively little radiant energy is wasted through absorption into the atmosphere. Furthermore, radiant energy may be directed with reflectors and the like so that most of it is received and absorbed by the printed material in a dryer.
Yet another advantage of using radiant energy for drying printed materials relates to the control and elimination of pollutants found in the evaporated solvents, varnishes and pigments of inks and dyes. Prior art forced air-type convection dryers which include burners for natural gas, propane or fuel oil generally emit substantial amounts of pollutants in the form of evaporated solvents, varnishes and pigments. Accordingly, air pollution control equipment such as afterburners for thermal and catalytic incineration and electrostatic precipitators for particulates must be provided in order to satisfy applicable environmental protection laws and regulations and to meet local code requirements. However, a large portion of the aforementioned pollutants from a drying operation may be incinerated within the dryer or otherwise converted to a less harmful condition by radiant energy. Therefore, dryers which employ radiant energy require less pollution control equipment than convection-type dryers of comparable capacity or else require no pollution control equipment.
In particular, short and medium wave infrared energy with wavelengths of 0.75 to 1.5 and 1.5 to 3.0 microns respectively is effective for drying and incinerating the solvents, varnishes and pigments in printing inks and dyes. The wavelength of infrared radiation is determined by the temperature of the emitting body with higher temperatures producing shorter wavelengths. Short and medium wave infrared radiation penetrates deeper and is more intense than long wave infrared radiation. Although all bodies not at absolute zero emit infrared radiation, quartz tube heat lamps offer several advantages. First of all, they are relatively efficient in converting input electricity into radiant heat for drying ink and dye solvents, varnishes and pigments with minimal heat loss through absorption into the ambient atmosphere. For example, quartz tube heat lamps are available with an operational efficiency of approximately 89%.
Secondly, quartz tube heat lamps tend to have a relatively low thermal mass and thus can be switched on and off relatively rapidly. In drying wet materials, this is particularly important as a safety feature because in the event of a web stoppage, it is important to remove the heat therefrom immediately to avoid igniting the stopped web.
Furthermore, the output of quartz tube heat lamps can be relatively precisely controlled by varying the electrical input to their filaments. Thus, a web of printed material may be run at a constant speed through a dryer or oven and the precise amount of radiant heat required for proper drying can be applied by such control means. Drying systems lacking such precise control means are not only wasteful of energy, but may actually harm the printed product. For example, if the ink is not sufficiently dry, it is more susceptible to smearing. On the other hand, excessive heat can weaken the cellulose in the printed material, break the web or ignite it.
To take advantage of the aforementioned characteristics of infrared heating, many modern printing systems employ inks which are especially formulated to dry quickly when exposed to infrared radiant energy.
The drying of ink and dye and solvents on printed materials may also be hastened by directing a drying fluid such as air, oxygen or ozone over the printed material. Without such a fluid flow, heat-induced drying is less efficient because the evaporated solvent tends to collect in a layer over the printed material. The evaporated solvent insulates the printed material from radiant and convection heat and inhibits further solvent evaporation. Although movement of a web of printed material accounts for a certain amount of air circulation, a system for forcing drying fluid through the dryer in conjunction with a heat source is preferred.
Quartz tube heat lamps and forced drying fluid systems are known in the printing art as exemplified by the Hanson U.S. Pat. No. 2,065,070; the Early et al. U.S. Pat. No. 3,159,464 and the Visser U.S. Pat. No. 3,122,999. However, a persistent problem not effectively solved by the prior art relates to the immediate removal of the radiant heat from the web in the event of a stoppage for any reason. If the heat is not removed quickly enough, the web may be scorched, ignited and broken. Although quartz tube heat lamps have relatively low thermal masses as aforementioned, the high operating temperatures required to produce short wave infrared radiation increase the risk of web damage if the heat lamps are not immediately withdrawn a safe distance from the stopped web. Also, many prior art dryers were ineffective at reducing the level of pollutants in their exhausts.
Heretofore quartz tube heat lamps and a forced air system have not been available in a dryer designed to reduce the exhaust pollutant level and to automatically remove the heat source from the printed material as required to safely operate the heat lamps in the short wave infrared range.